Instagram for iOS has been updated to support the new screen resolutions on the iPhone XS Max and iPhone XR. Version 65 is optimized for the larger screen and the new resolution that allows you to view slightly more content on the larger screen.
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For version 65, Instagram (and other apps that were not yet modified) simply showed a shortened version of the iPhone X version of the app.
This has no effect on the iPhone XS that has the same 5.8-inch screen and the same resolution as the iPhone X, but both the iPhone XS Max and the iPhone XR have larger 6.5-inch and 6.1-inch screens with higher resolution. Apps that have not yet been optimized are not a big thorn in the eye, but they are visually less sharp than on the new screens.
If you prefer the scaled up version for accessibility benefits, you can enable Display Zoom on iPhone XS Max and iPhone XR.
Instagram version 65 with support for iPhone XS Max and iPhone XR resolution is available in the App Store.